Various methods for transmitting high-quality and high-volume data under limited radio resources has been developed in wireless communication systems. A closed-loop system for providing a feedback channel from a user equipment to a base station is one of the methods.
In the closed-loop system, the user equipment provides various types of information to the base station and the base station adjusts a variety of system parameters, such as a power level, transmission format, and the like, using the provided information to improve system performance. For example, an adaptive modulation and coding (AMC) is a technique in which the base station adjusts a modulation and coding scheme (MCS) using feedback channel information to enhance link performance. If channel status is good, a data transmission rate is increased, whereas if the channel status is degraded, the data transmission rate is decreased, thereby supporting efficient transmission and increasing an average transmission rate as a result.
Channel quality information (CQI) is an example of the feedback information. Generally, CQI is measured at the user equipment and transmitted to the base station through an uplink channel. The base station can allocate best resources to each of the user equipment through the CQI.
In addition, various types of information, such as band selection information, acknowledgement/not-acknowledgement (ACK/NACK) signals, rank indicator (RI), precoding matrix indicator (PMI), and the like, as well as the CQI, are included in the feedback information transmitted from the user equipment to the base station. As various types of feedback information are required, overheads can be incurred in the uplink.
If five bits (25=32 levels) are used to transmit CQI, the CQI can be expressed in a quantum of between −10 dB and −22 dB quantized at intervals of 1 dB. Alternatively, there are 32 types of MCS levels, and one of the MCS levels appropriate to the CQI can be expressed with five bits. The five bits are transmitted after being channel coded, and since the feedback information should be further reliably coded than general uplink data, a high code rate is used. Therefore, a wide uplink bandwidth is used to transmit the feedback information. In this case, the bandwidth to be used for transmitting general uplink data will be decreased.
The entire frequency bandwidth is divided into 25 resource blocks (RBs), and one CQI can be transmitted through each of the resource blocks. In this case, as many as ‘25×5×code rate’ bits are required for each user equipment to feedback CQI of five bits. Alternatively, the CQI can be transmitted through some of the resource blocks showing excellent CQI in the feedback information. Even in this case, band selection information for indicating in which resource block the CQI belongs to is separately needed.
In addition, a system using multiple antennas needs an RI. If a precoding method is used, a PMI is needed to indicate which precoding matrix is used.
As described above, the number of bits to be transmitted varies depending on the type of feedback information, and a different channel coding scheme is needed for feedback information of a different length, and thus the system can be complicated. Furthermore, an identifier for distinguishing a type of feedback information from various types of feedback information should be separately transmitted.
Accordingly, there is a need for a method for reducing overheads incurred by transmission of various types of feedback information and transmitting reliable feedback information through a narrow uplink bandwidth.